Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase.
Methods for the chemical synthesis of 1-butanol are known, such as the Oxo Process, the Reppe Process, and the hydrogenation of crotonaldehyde (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719). These processes use starting materials derived from petrochemicals and are generally expensive and are not environmentally friendly. The production of 1-butanol from plant-derived raw materials would minimize green house gas emissions and would represent an advance in the art.
Methods for producing 1-butanol by biotransformation of other organic chemicals are also known. For example, Muramoto et al. (JP63017695) describe a method for the production of alcohols, including butanol, from aldehydes using strains of Pseudomonas. Additionally, Kuehnle et al. (EP 1149918) describe a process for preparing 1-butanol and 2-butanol by the oxidation of hydrocarbons by various strains of Rhodococcus ruber. 
Methods of producing butanol by fermentation are also known, where the most popular process produces a mixture of acetone, 1-butanol and ethanol and is referred to as the ABE processes (Blaschek et al., U.S. Pat. No. 6,358,717). Acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum is one of the oldest known industrial fermentations, and the pathways and genes responsible for the production of these solvents have been reported (Girbal et al., Trends in Biotechnology 16:11-16 (1998)). The actual fermentation, however, has been quite complicated and difficult to control. ABE fermentation has declined continuously since the 1950s, and almost all butanol is now produced via petrochemical routes, as described above. In a typical ABE fermentation, butyric, propionic, lactic and acetic acids are first produced by C. acetobutylicum, the culture pH drops and undergoes a metabolic “butterfly” shift, and 1-butanol, acetone, isopropanol and ethanol are then formed. In conventional ABE fermentations, the 1-butanol yield from glucose is low, typically around 15 percent and rarely exceeding 25 percent. Consequently, the 1-butanol concentration in conventional ABE fermentations is usually lower than 1.3 percent.
Attempts to maximize 1-butanol production from the ABE process by the elimination of all the other solvent by-products have not been totally successful, and thus, the process produces significant amounts of acetone which is not useful as a gasoline additive. A process for the fermentive production of butanol where 1-butanol is the sole product would represent an advance in the art.
There is a need, therefore, for an environmentally responsible, cost-effective process for the production of 1-butanol as a single product. The present invention addresses this need through the discovery of a recombinant microbial production host expressing a 1-butanol biosynthetic pathway.